Name derivation:

“Many leaves” (Many Myrio- leaves –phyllum)

Morphology:

Whorled leaves (4-5) are pinnatifid, often described as similar to bird
feathers.Foliage submerged except on
floral stems that differentiate from underwater sections by enlarging their girth
to increase flotation, bend upward, and grow above the water surface by ~ 10
cm.Aerial leaves are reduced in size
and approach margins that are nearly ‘entire’ but
with small ‘teeth’.In northern
ice-covered lakes most of the foliage ‘dies back’ leaving mainly the roots
intact for growth after ice-out.

Roots are fibrous, extending 10 –
15 cm into organically-rich lake sediments.Adventitious roots can form at nodes along the stem.Asexual proliferation of plants can occur
either from intact stems weighed down by epiphytes and acting as ‘runners’
along the sediment, or by fragmentation of the fragile stems followed by
drifting and sinking.

Dwarf plants can be found growing
in moist sandy substrates, with aerial leaves.If subsequently submerged the plants
develop leaves typical of their underwater foliage.

Because of morphological plasticity
of leaves, flower presence is essential for identifying species.

Similar genera:

Because of its whorled leaves several other genera
superficially are similar even though their leaf morphology differs
significantly. These include submerged plants such as Elodea, Egeria
and Hydrilla,
as well as Ceratophyllum.
All of these can form extensive 'canopies'.

Habitat:

Various species can grow in a variety of aquatic
habitats from well-buffered calcium-rich waters to poorly buffered somewhat
acidic lakes.M. spicatum,
considered invasive from South America into the USA, seems to prefer
well-buffered waters. M. heterophyllum is considered native to Quebec and
Ontario and NE US states (Godfrey and Wooten 1981),
and proliferates in somewhat acidic lakes.It is considered ‘naturalised’ in
Europe.The species is even considered
to be ‘endangered’ in Ohio (online)

References:

Godfrey, R.K.,
and J.W. Wooten1981.Aquatic and Wetland Plants of the
Southeastern United States.Dicotyledons.University
of Georgia Press. (712 pp.)